1. Field of the Invention
This invention relates in general to devices for indicating the depth of a hole drilled by an earth boring machine, particularly of the type utilizing a hydraulic cylinder for raising and lowering the drilling tool.
2. Description of the Prior Art
One type of machine for drilling holes for foundations, utility poles, and the like, utilizes a hydraulic cylinder for moving an auger in and out of the hole. A piston is located in the cylinder and connected to a kelly for moving the kelly longitudinally in response to hydraulic fluid pressure. An auger or other type of drilling tool is connected to the bottom of the kelly, which is rotatably driven. An example of this type drilling machine is disclosed in U.S. Pat. No. 3,525,404, "Rotary Drilling Rig With Direct Power Drive and Simplified Controls", E. F. Newman et al., Aug. 25, 1970.
It is advantageous to know the hole depth during drilling. The operator has to pull the auger from the hole after drilling every two or so feet in order to spin spoil from the auger flights. When returning the auger through the hole, damage can result if the bottom of the hole is struck too forcefully. Such accidents can be avoided if the operator has knowledge of the depth of the hole and the instantaneous locations of the end of the auger.
Since the kelly is enclosed in the hydraulic cylinder, the depth of the hole is difficult to detect visually. There is one patented depth measuring system in a hydraulically operated drilling machine, U.S. Pat. No. 3,651,871, "Drilling Rig Depth Control", Palmer G. Greene, Mar. 28, 1972. In this system depth is monitored by recording the length traveled by the drill pipe supporting member, comparing the length traveled with a predetermined length, and producing a signal when the two are equal. The signals are utilized to determine the total length of the drill pipe in the hole.